A variable capacitance element generally has such a structure in which a stationary or fixed electrode and a movable electrode are disposed facing each other, and the capacitance is varied by displacing the movable electrode. The movable electrode can be displaced by piezoelectric drive, electrostatic drive, etc. In mobile electronic devices, miniaturization and reduction of weight are required, and variable capacitance elements using MEMS (micro electro-mechanical system) are being developed.
Such a structure is known wherein a stationary or fixed electrode is formed on a support substrate, a movable electrode is supported by a flexible beam, etc. above the stationary electrode, and the capacitance is varied by controlling the distance between the electrodes (for example, see JP-A 2006-147995).
FIG. 7A illustrates a structural example of such variable capacitance element. A variable capacitance element is formed by a variable capacitor with parallel plate structure one electrode of which is made movable, and a container structure sealing this variable capacitor.
A stationary electrode 103 and anchors 106 are formed on a semiconductor substrate 101 of such as silicon, via an insulating layer 102. The anchors 106 support a plate-shaped movable electrode 104 above the stationary electrode 103 via U-shaped flexible beams 105. The container including sidewall 110 and ceiling 111 is formed to surround outer periphery of the variable capacitor. By the existence of this container, it becomes possible to seal the variable capacitor in an inert gas atmosphere such as rare gas, or in a reduced pressure atmosphere. When the container is made of metal material, electric shield also becomes possible.
When voltage V is applied between the stationary electrode 103 and the movable electrode 104, the movable electrode 104 is attracted toward the statinary electrode 103 by the electrostatic force. When the movable electrode 104 is displaced toward the stationary electrode 103, the flexible beams 105 are bent. Restoring force proportional to the amount of displacement works to return the movable electrode 104 back to the original position. The movable electrode 104 is displaced up to the balanced position where the electrostatic force and the restoring force balance each other, and is held at the balanced position as long as the voltage V is applied.
When the voltage V is reduced to zero, the movable electrode 104 returns to the original position. Therefore, the capacitance element constituted of the stationary electrode 103 and the movable electrode 104, works as a variable capacitance element the static capacitance of which can be controlled by the applied voltage V.
FIG. 7B is a cross-section of another structural example of variable capacitance element. On a semiconductor substrate 101 of such as silicon, a stationary or fixed electrode 103 is formed through an insulating layer 102, and another insulating layer 112 is formed on the insulating layer 102 to cover the stationary electrode 103. Anchors 106 are formed on the insulating layer 112. The anchors 106 support a plate shaped movable electrode 104 via flexible beams 105 above the stationary electrode 103 via the insulating layer 112. A container including sidewall 110 and ceiling 111 is formed to surround the outer periphery of the variable capacitor. Since the surface of the stationary electrode 103 is covered with the insulating layer 112, short-circuit and sticking between the electrodes can be suppressed.
In a digital type variable capacitor element, capacitance formed in a state where the movable electrode is separated from the fixed electrode, is the minimum value (off state), and capacitance formed in a state where the movable electrode touches the fixed electrode through a dielectric film, is the maximum value (on state). These two states are used as a variable capacitance.
The electrode of a capacitor can be formed not only parallel to the substrate surface, but also be formed perpendicular or vertical to the substrate surface (for example, see JP-A 2001-304868)). For example, a variable capacitor having electrodes perpendicular to the substrate surface can be formed using an SOI (silicon-on-insulator) substrate in which a single crystal silicon layer is provided above upper surface of a single crystal silicon substrate via a silicon oxide film serving as a binding layer.
Impurity atoms such as phosphor and boron are doped in the single crystal silicon layer to reduce the resistance of the single crystal silicon layer. A resist mask is formed on the single crystal silicon layer, and the single crystal silicon layer is etched by reactive ion etching, etc. leaving anchors, various comb shaped electrodes, and various pad portions on the silicon oxide film. The comb shaped electrodes are coupled in inter digital shape to form a capacitor. The respective electrodes are shaped perpendicular to the silicon substrate surface.
The silicon oxide film can be removed by selective etching by frolic acid aqueous solution, etc. to separate the active silicon layer from the support Si substrate, to give freedom of displacement. Such structures as vibrators, beams, and comb shaped electrodes can be formed. Conductor such as aluminum is vapor deposited on various pad portions to form electrode pads. Such a structure is obtained in which respective portions formed above the substrate are constituted of low resistivity layers insulated from the substrate, and vibrators, beams, comb shaped electrodes etc. are positioned floating above the substrate by a predetermined distance, and are supported by the substrate to be capable of vibration via the anchors.
Patent document 1: JP-A 2006-147995,
Patent document 2: LP-A 2001-304868